The number on the dust cover and the guard is the Army & Navy number (and they match) and from my limited research the number stamped on the underside of the bolt handle is the BSA number. I have found pictures of other rifles with the 4 digit alpha numeric code but no explanation as yet. I have an enquiry in with Glasgow University who apparently have the Army & Navy CSL records (Through the House of Fraser). Will be interesting to see what may come back. Thanks